Wafer alignment for small-angle X-ray scatterometry

ABSTRACT

An X-ray apparatus includes a mount, an X-ray source, a detector, an optical gauge and a motor. The mount is configured to hold a planar sample having a first side, which is smooth, and a second side, which is opposite the first side and on which a pattern has been formed. The X-ray source is configured to direct a first beam of X-rays toward the first side of the sample. The detector is positioned on the second side of the sample so as to receive at least a part of the X-rays that have been transmitted through the sample and scattered from the pattern. The optical gauge is configured to direct a second beam of optical radiation toward the first side of the sample, to sense the optical radiation that is reflected from the first side of the sample, and to output a signal, in response to the sensed optical radiation, that is indicative of a position of the sample. The motor is configured to adjust an alignment between the detector and the sample in response to the signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/661,133, filed Apr. 23, 2018, whose disclosure isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to X-ray analysis, andparticularly to methods and systems for measuring geometrical structuresof semiconductor devices using X-ray scatterometry.

BACKGROUND OF THE INVENTION

X-ray scatterometry techniques are used for measuring geometricalstructures of semiconductor devices.

For example, U.S. Pat. No. 7,481,579 describes a method for inspectionthat includes directing a beam of X-rays to impinge upon an area of asample containing first and second features formed respectively in firstand second thin film layers, which are overlaid on a surface of thesample. A pattern of the X-rays diffracted from the first and secondfeatures is detected and analyzed in order to assess an alignment of thefirst and second features.

U.S. Pat. No. 9,606,073 describes apparatus that includes asample-support that retains a sample in a plane having an axis, theplane defining first and second regions separated by the plane. Asource-mount in the first region rotates about the axis, and an X-raysource on the source-mount directs first and second incident beams ofX-rays to impinge on the sample at first and second angles along beamaxes that are orthogonal to the axis. A detector-mount in the secondregion moves in a plane orthogonal to the axis and an X-ray detector onthe detector-mount receives first and second diffracted beams of X-raystransmitted through the sample in response to the first and secondincident beams, and outputs first and second signals, respectively, inresponse to the received first and second diffracted beams. A processoranalyzes the first and the second signals so as to determine a profileof a surface of the sample.

U.S. Pat. No. 6,895,075 describes apparatus for inspection of a sample,the apparatus includes a radiation source and an array of detectorelements arranged to receive radiation from a surface of the sample dueto irradiation of an area of the surface by the radiation source.

U.S. Pat. No. 7,551,719 describes apparatus for analysis of a sample,the apparatus includes a radiation source, which is adapted to direct afirst, converging beam of X-rays toward a surface of the sample and todirect a second, collimated beam of the X-rays toward the surface of thesample. A motion assembly moves the radiation source between a firstsource position, in which the X-rays are directed toward the surface ofthe sample at a grazing angle, and a second source position, in whichthe X-rays are directed toward the surface in a vicinity of a Braggangle of the sample.

U.S. Pat. No. 8,243,878 describes a method for analysis includingdirecting a converging beam of X-rays toward a surface of a samplehaving an epitaxial layer formed thereon, and sensing the X-rays thatare diffracted from the sample while resolving the sensed X-rays as afunction of angle so as to generate a diffraction spectrum including adiffraction peak and fringes due to the epitaxial layer.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesan X-ray apparatus that includes a mount, an X-ray source, a detectorand a beam limiter. The mount is configured to hold a planar sample. TheX-ray source is configured to direct a beam of X-rays toward a firstside of the sample. The detector is positioned on a second side of thesample, opposite the first side, so as to receive at least a part of theX-rays that have been transmitted through the sample. The beam limiteris positioned on the first side of the sample so as to intercept thebeam of the X-rays. The beam limiter includes first and second bladesand first and second actuators. The first and second blades haverespective first and second edges positioned in mutual proximity so asto define a slit, through which the beam of the X-rays will pass, at adistance smaller than 25 mm from the first side of the sample. The firstand second actuators are configured to shift the first and second bladesalong respective, first and second translation axes so as to adjust awidth of the slit.

In some embodiments, the mount is configured to tilt the sample about atilt axis in a plane of the sample, and the slit is oriented parallel tothe tilt axis. In other embodiments, the first and second blades includea material made from a single-crystalline material or a polycrystallinematerial. In yet other embodiments, the first and second blades are notparallel to one another.

In an embodiment, the first and second translation axes are not parallelto one another. In another embodiment, the beam limiter is configured tocontrol at least one beam parameter, selected from a list consisting of(i) a position of the beam, (ii) a spot size of the beam, (iii) a spotshape of the beam on the first side of the sample, and (iv) aconvergence or divergence angle of the beam. In yet another embodiment,the beam limiter is mounted on a stage, which is configured to move thebeam limiter relative to at least one of the beam and the sample.

In some embodiments, the stage includes at least a rotation stage. Inother embodiments, at least one of the first and second actuatorsincludes one or more piezoelectric linear motors. In yet otherembodiments, the apparatus includes first and second movable plateshaving respective first and second plate edges positioned in mutualproximity so as to define an additional slit, through which the beam ofthe X-rays will pass, before or after passing through the slit.

In an embodiment, the apparatus includes a third actuator, which isconfigured to shift at least one of the first and second movable platesalong a third translation axis so as to adjust a size of the additionalslit. In another embodiment, the apparatus includes a processor, whichis configured to shape the beam of the X-rays by aligning a position ofthe slit and the additional slit relative to one another.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method including holding a planar sample on amount. A beam of X-rays is directed from an X-ray source toward a firstside of the sample. At least a part of the X-rays that have beentransmitted through the sample are received from a detector positionedon a second side of the sample, opposite the first side. A beam limiteris positioned on the first side of the sample so as to intercept thebeam of the X-rays. The beam limiter includes first and second blades,having respective first and second edges positioned in mutual proximityso as to define a slit, through which the beam of the X-rays will pass,at a distance smaller than 25 mm from the first side of the sample, andfirst and second actuators. A width of the slit is adjusted by shiftingthe first and second blades along respective first and secondtranslation axes, using the first and second actuators.

There is further provided, in accordance with an embodiment of thepresent invention, an X-ray optical device that includes a crystal, anX-ray mirror and one or more slits. The crystal contains a channelhaving an entrance aperture, an exit aperture, and opposing internalfaces arranged so that the channel tapers from the entrance aperture tothe exit aperture. The X-ray mirror includes a curved substrate with amultilayer coating, which is configured to collect and direct a beam ofX-rays emitted from a source into the entrance aperture of the channelwith a first beam diameter, so that the beam is emitted from the exitaperture with a second beam diameter, less than the first beam diameter.One or more slits interposed between the X-ray mirror and the crystal,so that the beam passes through the slits before entering the entranceaperture of the channel.

In some embodiments, the X-ray mirror is configured to adjust adivergence and intensity of the beam. In other embodiments, the deviceincludes a blade and an actuator. The blade has an array of apertures ofdifferent, respective sizes passing therethrough. The actuator isconfigured to position the blade in a path of the beam emitted from theexit aperture of the crystal and to translate the blade so as toposition different ones of the aperture in the path.

In an embodiment, the crystal includes a single-crystal made fromgermanium. In another embodiment, the opposing internal faces are notparallel to one another.

There is additionally provided, in accordance with an embodiment of thepresent invention, an X-ray apparatus that includes a mount, an X-raysource, a detector, an optical gauge and a motor. The mount isconfigured to hold a planar sample having a first side, which is smooth,and a second side, which is opposite the first side and on which apattern has been formed. The X-ray source is configured to direct afirst beam of X-rays toward the first side of the sample. The detectoris positioned on the second side of the sample so as to receive at leasta part of the X-rays that have been transmitted through the sample andscattered from the pattern. The optical gauge is configured to direct asecond beam of optical radiation toward the first side of the sample, tosense the optical radiation that is reflected from the first side of thesample, and to output a signal, in response to the sensed opticalradiation, that is indicative of a position of the sample. The motor isconfigured to adjust an alignment between the detector and the sample inresponse to the signal.

In some embodiments, the signal is indicative of at least one positionparameter, selected from a group of position parameters consisting of adistance between the sample and the detector and an orientation of thesample relative to the detector. In other embodiments, the orientationof the sample includes an inclination angle of the sample relative to asurface of the detector. In yet other embodiments, the sample includes asingle-crystal material, and the X-ray apparatus includes an additionaldetector, which is configured to measure an intensity of at least aportion of the X-rays that has been diffracted from a lattice plane ofthe single-crystal material, and the X-ray apparatus further includes acontroller, which is configured to calibrate an orientation of the firstbeam of X-rays with respect to the lattice plane responsively to themeasured intensity.

In an embodiment, the X-ray apparatus includes a processor, which isconfigured to instruct the optical gauge to direct the second beamtoward multiple locations on the first side of the sample so as tooutput multiple respective signals indicative of multiple respectiveoptical radiations reflected from the multiple locations. The processoris further configured to display, based on the multiple signals, athree-dimensional (3D) map indicative of the position of the sample atleast at the multiple locations. In another embodiment, the processor isconfigured to estimate, based on the multiple locations, one or moreadditional positions of the sample at additional one or more respectivelocations on the first side, and to display the additional locations onthe 3D map.

In some embodiments, the X-ray apparatus includes an energy dispersiveX-ray (EDX) detector assembly, which is configured to measure X-rayfluorescence emitted from the pattern at the position of the sample, andto output an electrical signal indicative of an intensity of the X-rayfluorescence measured at the position. In other embodiments, the EDXdetector assembly includes a silicon-based or a germanium-basedsolid-state EDX detector.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method including holding, on a mount, a planarsample having a first side, which is smooth, and a second side, which isopposite the first side and on which a pattern has been formed. A firstbeam of X-rays is directed toward the first side of the sample. At leasta part of the X-rays that have been transmitted through the sample andscattered from the pattern is received from a detector positioned on thesecond side of the sample. A second beam of optical radiation isdirected toward the first side of the sample for sensing the opticalradiation that is reflected from the first side of the sample, and asignal that is indicative of a position of the sample, is output inresponse to the sensed optical radiation. An alignment, between thedetector and the sample, is adjusted in response to the signal.

There is further provided, in accordance with an embodiment of thepresent invention, an X-ray apparatus that includes a mount, an X-raysource, a detector, a motor, and a controller. The mount is configuredto hold a sample that includes a single-crystal material and has a firstside and a second side, which is opposite the first side. The X-raysource is configured to direct a beam of X-rays toward the first side ofthe sample. The detector is positioned on the second side of the sampleand is configured to receive at least a portion of the X-rays that havebeen diffracted from a lattice plane of the single-crystal material. Themotor is configured to adjust an alignment between the detector and thesample. The controller is configured to measure an orientation of thesample relative to the detector based on the diffracted X-rays and todrive the motor to adjust the alignment responsively to the measuredorientation.

There is additionally provided, in accordance with an embodiment of thepresent invention, an X-ray apparatus that includes a mount, an X-raysource, a detector, an actuator, and a controller. The mount isconfigured to hold a sample. The X-ray source is configured to direct abeam of X-rays toward a first side of the sample. The detector ispositioned on a second side of the sample, opposite the first side, soas to receive at least a portion of the X-rays that have beentransmitted through the sample and to output signals indicative of anintensity of the received X-rays. The actuator is configured to scan thedetector over a range of positions on the second side of the sample soas to measure the transmitted X-rays as a function of a scatteringangle. The controller is coupled to receive the signals output by thedetector and to control the actuator, responsively to the signals, so asto increase an acquisition time of the detector at first positions wherethe intensity of the received X-rays is weak relative to the acquisitiontime at second positions where the intensity of the received X-rays isstrong.

In some embodiments, the detector includes an array of sensor elementshaving a predefined pitch, and the actuator is configured to step thedetector across the range of positions with a resolution that is finerthan the predefined pitch. In other embodiments, the array includes atwo-dimensional matrix of the sensor elements, and the actuator isconfigured to step the detector with the resolution that is finer thanthe pitch along both height and width axes of the matrix.

In an embodiment, the sample includes one or more high aspect ratio(HAR) features having an aspect ratio larger than ten, and the actuatoris configured to scan the detector over the range of positions so as tomeasure the transmitted X-rays scattered from the HAR features. Inanother embodiment, the controller is configured to control theacquisition time so that the detector receives a predefined intensityrange at the first and second positions.

There is also provided, in accordance with an embodiment of the presentinvention, a method including holding a sample on a mount. A beam ofX-rays is directed toward a first side of the sample. At least a portionof the X-rays that have been transmitted through the sample is receivedfrom a detector positioned on a second side of the sample, opposite thefirst side, and signals indicative of an intensity of the receivedX-rays are output. The detector is scanned, by an actuator, over a rangeof positions on the second side of the sample so as to measure thetransmitted X-rays as a function of a scattering angle. The signalsoutput are received by the detector and the actuator is controlled,responsively to the signals, so as to increase an acquisition time ofthe detector at first positions where the intensity of the receivedX-rays is weak relative to the acquisition time at second positionswhere the intensity of the received X-rays is strong.

There is additionally provided, in accordance with an embodiment of thepresent invention, an X-ray apparatus that includes a first mount, anX-ray source, a detector, and a beam blocker. The first mount isconfigured to hold a sample. The X-ray source is configured to direct abeam of X-rays toward the sample. The detector is positioned to receivethe X-rays that have been transmitted through the sample, at least partof the transmitted beams are scattered from the sample over a range ofangles. The beam blocker includes a second mount made of a material thatis transparent to the X-rays and one or more pieces of an X-ray opaquematerial held within the second mount, and is positionable so that theX-ray opaque material blocks the X-rays in a part of the range ofangles, while the X-rays at the angles surrounding the blocked part ofthe range pass through the mount to the detector.

In some embodiments, at least one of the pieces of the X-ray opaquematerial is ellipsoidal. In other embodiments, the mount includes apolymer. In yet other embodiments, the mount includes diamond.

In an embodiment, at least part of the blocked X-rays include X-raystransmitted through the sample without being scattered. In anotherembodiment, the X-ray apparatus includes a processor, which isconfigured to measure an intensity of the X-rays received by thedetector, and to position the beam blocker relative to the transmittedbeam responsively to the measured intensity. In yet another embodiment,at least one of the pieces of the X-ray opaque material is held within arecess of the mount.

In some embodiments, the mount includes a sheet made from (i)biaxially-oriented polyethylene terephthalate (BoPET) polyester, or (ii)poly (4,4′-oxydiphenylene-pyromellitimide) polyimide. In otherembodiments, at least one of the pieces of the X-ray opaque materialincludes gold, tantalum, or tungsten. In yet other embodiments, thepieces of the X-ray opaque material include at least a first piece and asecond piece, having a different size and laid out in an array at apredefined distance from one another.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are schematic illustrations of small-angle X-ray scattering(SAXS) systems, in accordance with embodiments of the present invention;

FIG. 4 is a schematic illustration of a beam conditioning assembly, inaccordance with an embodiment of the present invention;

FIGS. 5 and 6 are schematic illustrations of slit assemblies, inaccordance with embodiments of the present invention;

FIGS. 7A and 7B are schematic illustrations of beam blocking assemblies,in accordance with embodiments of the present invention;

FIG. 8A is a schematic illustration of an image indicative of theintensity of an X-ray beam sensed by a detector without a beam blocker,in accordance with another embodiment of the present invention;

FIG. 8B is a schematic illustration of an image indicative of theintensity of an X-ray beam sensed by a detector in the presence of abeam blocker, in accordance with an embodiment of the present invention;

FIG. 9A is a schematic illustration of an image indicative of theintensity of a scattered X-ray beam sensed by a detector without a beamblocker, in accordance with another embodiment of the present invention;

FIG. 9B is a schematic illustration of an image indicative of theintensity of a scattered X-ray beam sensed by a detector in the presenceof beam blocker, in accordance with an embodiment of the presentinvention; and

FIG. 10 is a schematic illustration of a scanning scheme in which anX-ray detector comprising an array of sensors is moved at steps smallerthan the inter-distance of the sensors, for improved angular resolution,in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments of the present invention that are described hereinbelowprovide improved methods and systems for analyzing geometrical featuresformed in various types of semiconductor devices and test structures.X-ray scatterometry techniques for analyzing features, such assmall-angle X-ray scattering (SAXS) methods, typically apply X-rayswhose wavelengths are on the order of one angstrom. Such wavelengths aresuitable for measuring High Aspect Ratio (HAR) features such as HARholes or trenches fabricated in semiconductor wafers. Measuringgeometrical and other properties of the features is carried out based onanalyzing the intensities of the X-rays scattered from the wafer atvarious angles.

In some embodiments, a SAXS system comprises a motorized stage, which isconfigured to move a planar sample, such as a wafer having front andback surfaces facing one another, wherein the front surface comprisesvarious types of features, such as HAR features. Additionally oralternatively, the back surface of the wafer may be patterned withsimilar and/or other types of features.

In some embodiments, the SAXS system comprises an X-ray source, which isconfigured to direct a beam of X-rays toward the back surface of thewafer. The SAXS system further comprises at least one detector, facingthe front surface of the wafer, the detector is configured to sense atleast part of the X-rays that have been scattered from and/ortransmitted through the wafer. The detector is configured to produceelectrical signals indicative of the intensity of the X-rays scatteredfrom HAR features in the front surface of the wafer, and received by thedetector.

In some embodiments, the SAXS system comprises a processor, which isconfigured to measure properties of the HAR features in question, basedon the electrical signals received from the detector.

In some embodiments, the SAXS system comprises a beam conditioningassembly, positioned between the X-ray source and the back surface ofthe wafer, and configured to adjust properties of the X-ray beam. Thebeam conditioning assembly comprises a crystal containing a v-shapedchannel having an entrance aperture, an exit aperture, and opposinginternal faces arranged so that the channel tapers from the entranceaperture to the exit aperture. The beam conditioning assembly furthercomprises an X-ray mirror, having a curved substrate with a multilayercoating. The mirror is configured to collect the beam and direct thecollected beam into the entrance aperture of the channel with a firstbeam diameter, so that the beam that is emitted from the exit aperturehas a second beam diameter, smaller than the first beam diameter.

In some embodiments, the SAXS system comprises a first slit, which ispositioned between the X-ray source and the back surface of the wafer soas to intercept the beam and to adjust spatial properties of theintercepted beam. The first slit comprises first and second movableblades that are typically not parallel to one another. The edges of thefirst and second blades are positioned in close proximity to one anotherso as to define the slit. In some embodiments, the processor isconfigured to move the edges of the first and second blades so as tocontrol spatial properties of the beam by adjusting the width of theslit.

In alternative embodiments, the SAXS system comprises a second slit,positioned between the X-ray source and the back surface of the wafer.The second slit comprises a movable blade having multiplescatterless-pinholes, each of which having a different width. Theprocessor is configured to position a selected scatterless-pinhole tointercept the beam by moving the movable blade, so as to control thespatial properties of the beam.

In some embodiments, the SAXS system comprises an optical gauge, whichis configured to direct a light beam toward the back side of the wafer,to sense the optical radiation reflected therefrom using a detector,and, in response to the sensed optical radiation, to output, by thedetector, a signal that is indicative of the position of the wafer.Based on the signal, the processor is configured to estimate positionparameters, such as the distance between the wafer and the detector, andthe orientation of the wafer relative to the detector. The SAXS systemfurther comprises a motor, which is controlled by the processor so as toalign the orientation between the X-ray beam and the wafer in responseto the signal.

In some embodiments, the wafer comprises single-crystalline material,and the detector is configured to measure one or more beams diffractedfrom a lattice plane of the single crystal. The SAXS system furthercomprises a controller, which is configured to calibrate the position ofthe optical gauge relative to the lattice plane in response to themeasured diffraction. Based on the diffracted X-rays, the controller isfurther configured to measure the orientation of the wafer relative tothe detector, and to drive at least one motor to align the orientationbetween the wafer and the incident X-ray beam, based on the measuredorientation. In other embodiments, the processor may carry out at leastsome of the operations described above, instead of the controller.

In some embodiments, the SAXS system comprises a detector mounted on oneor more actuators, which are configured to move the detector withrespect to the scattered X-rays, over a range of positions on the frontsurface of the wafer, so as to measure the intensities of thetransmitted X-rays as a function of scattering angle. This configurationallows to measure the intensities of the transmitted X-rays withincreased angular resolution than is possible by the native resolutionof the detector elements. In some embodiments, the processor isconfigured to control the actuator, in response to electrical signalsproduced by the detector, so that the acquisition time of the detectorinversely depends on the intensity of the sensed X-rays.

In some embodiments, the detector comprises a two-dimensional array(also referred to as a matrix) of sensor elements having a predefinedpitch along height and width axes of the matrix. The actuator isconfigured to step the detector across the range of positions at a finerresolution than the predefined pitch along both height and width axes.

In some embodiments, the SAXS system comprises a beam blocker having oneor more beam stoppers. The beam blocker comprises a mount made of amaterial that is transparent to the X-rays. The one or more beamstoppers are held within the mount, and are made from a material atleast partially opaque to the X-ray beam. The beam blocker may bepositioned so that the one or more beam stoppers block the X-rays in apart of the range of angles, whereas the X-rays at the anglessurrounding the blocked part of the beam, pass through the mount to thedetector. In an embodiment, at least one of the beam stoppers has anellipsoidal shape with smooth edge so as to prevent scattering of thebeam from the beam stopper.

The disclosed techniques improve the sensitivity of SAXS systems todetect small geometrical changes in HAR features, by improving theangular resolution at which the X-ray beams scattered from HAR featuresare sensed by the detector. Moreover, the disclosed techniques may beused for reducing the footprint of SAXS systems while maintainingmeasurements in high sensitivity and resolution.

System Description

FIG. 1 is a schematic illustration of a small-angle X-ray scattering(SAXS) system 10, in accordance with an embodiment of the presentinvention. In some embodiments, SAXS system 10, also refers to herein as“system 10” for brevity, is configured to measure features on a sample,in the present example, a wafer 190, using scatterometry techniques, aswill be described hereinbelow.

In some embodiments, wafer 190 may comprise any suitable microstructureor materials, such as a single-crystal, a poly-crystal, an amorphousmicrostructure or any suitable combination thereof, such as differentmicrostructures or materials at different locations of wafer 190.

In some embodiments, system 10 comprises an X-ray excitation source,referred to herein as a source 100, driven by a high-voltage powersupply unit (PSU) 26. In some embodiments, source 100 is configured toemit an X-ray beam 130, also referred to herein as “incident beam 130”or “beam 130” for brevity, having a suitable energy to pass throughwafer 190.

In some embodiments, source 100 is configured to generate an intenseX-ray emission having a wavelength equal to or smaller than 0.1 nm withan effective spot-size of about 150 μm or less.

In some embodiments, source 100 may comprise any suitable type ofhigh-brightness X-ray source, such as, but not limited to (a) a fixedsolid anode, (b) a rotating solid anode, (c) a liquid metal, or (d) asynchrotron.

In some embodiments, the fixed solid anode-based source comprises amicro-focus X-ray tube in which high-energy electrons (>=50 keV) in avacuum are incident with a molybdenum (Mo) or silver (Ag) anode or anyother suitable metallic element or alloy. Such micro-focus X-ray tubesare provided by multiple suppliers such as, but not limited to, IncoatecGmbH (Hamburg, Germany), or rtw RONTGEN-TECHNIK DR. WARRIKHOFF GmbH &Co. (Berlin, Germany).

In some embodiments, the rotating solid anode micro-focus X-ray sourcemay comprise a Mo or Ag anode or any other suitable metallic element oralloy. Suitable rotating anode X-ray sources are provided by multiplesuppliers, such as, Bruker AXS GmbH (Karlsruhe, Germany).

In some embodiments, the liquid metal X-ray source comprises an anode ina molten state. The anode may comprise any suitable one or more elementsor alloys, such as alloys of gallium (Ga) and indium (In). A suitableliquid metal X-ray source may be selected, for example, from one or moreof the MetalJet products offered by eXcillum AB (Kista, Sweden).

In some embodiments, a synchrotron-based source that comprise a compactelectron accelerator-based X-ray source, such as the those provided byLyncean Technologies (Fremont, Calif. 94539, USA) and others beingdeveloped by the scientific community.

In some embodiments, wafer 190 may comprise a semiconductor wafer havingsurfaces 191 and 192. In some embodiments, surface 191 comprises highaspect ratio (HAR) features produced, on surface 191 and/or into thebulk of wafer 190 or materials deposited thereon, using any suitablesemiconductor processes, such as deposition, lithography and etching.Note that in these embodiments, surface 192 typically remains flat andsmooth and does not comprise HAR structures or another pattern producedby lithography and etching. It will be understood that during theproduction of features on surface 191, some layers may be deposited as ablanket on some locations of surface 192, e.g., using chemical vapordeposition (CVD) processes, and may cause some unintended topography onsurface 192.

In other embodiments, at least part of surface 192 may be patterned withthe aforementioned HAR features and/or with any other suitable types offeatures. In alternative embodiments, only surface 192 may comprise theaforementioned HAR features.

In the context of the present disclosure, and in the claims, the term“aspect ratio” refers to an arithmetic ratio between the depth and width(e.g., diameter in the case of a circular hole), or between the heightand width of a given feature formed in wafer 190. Furthermore, the term“high aspect ratio (HAR)” typically refers to an aspect ratio higherthan 10. The HAR structures, also referred to herein as HAR features,may comprise various types of three-dimensional (3D) structures formed,for example, on a logic device (e.g., a microprocessor), or a NAND flashmemory device, or a dynamic random-access memory (DRAM) device, or onany other device.

In some embodiments, the HAR features may comprise one or more Finfield-effect transistors (FETs), gate-all-around (GAA) FETs, nanowireFETs of a complementary metal-oxide semiconductor (CMOS) device, anaccess transistor of a DRAM device, one or more channels of a 3D NANDflash device, one or more 3D capacitors of a DRAM device, or any othertype of HAR feature.

In some embodiments, system 10 comprises a computer 20, which comprisesa processor 22, an interface 24 and a display (not shown). Processor 22is configured to control various components and assemblies of system 10described below, and to process electrical signals received from amovable detector assembly, referred to herein as a detector 240.Interface 24 is configured to exchange electrical signals betweenprocessor 22 and the respective components and assemblies of system 10.

Typically, processor 22 comprises a general-purpose processor withsuitable front end and interface circuits, which is programmed insoftware to carry out the functions described herein. The software maybe downloaded to the processor in electronic form, over a network, forexample, or it may, alternatively or additionally, be provided and/orstored on non-transitory tangible media, such as magnetic, optical, orelectronic memory.

In some embodiments, beam 130 is emitted from source 100 and passesthrough a shutter and slit assembly of system 10, referred to herein as“assembly 110,” made from any suitable material opaque to X-rays. Insome embodiments, processor 22 is configured to set the position ofassembly 110 using one or more controlled actuators, such as motors orpiezoelectric-based drives (not shown).

In some embodiments, assembly 110 is configured to improve the usersafety of system 10 by blocking any X-ray radiation deflected from thedesigned optical path of beam 130. In some embodiments, processor 22 isconfigured to adjust the position and size of the slits, so as tocontrol the divergence and spatial shape of beam 130.

In some embodiments, system 10 comprises additional slits, controlled byprocessor 22 for adjusting the divergence, intensity and spot-size ofbeam 130, and for blocking undesired scattered radiation.

In some embodiments, system 10 comprises a beam conditioning assembly,referred to herein as “assembly 165,” whose structure is described indetail in FIG. 4 below. In some embodiments, assembly 165 comprisesoptical elements, such as a mirror 120 and slits 125. Mirror 120 isconfigured to collect beam 130 from source 100 and assembly 110 andshape the optical properties of beam 130. For example, mirror 120 isconfigured to produce a collimated beam or a focused beam, or acombination thereof (e.g., collimated in x-direction and focused iny-direction). Slits 125 are configured to adjust the properties of beam130, such as the divergence angle and the spot-size of the beam exitingmirror 120.

In some embodiments, beam conditioning assembly 165 may comprise avacuum chamber so as to prevent degradation of one or more of theaforementioned optical elements caused by the interaction between airand ionizing radiation on the surface of the optical elements.

In some embodiments, beam conditioning assembly 165 may have multipleconfigurations, some of which are described in detail in FIG. 4 below.For example, processor 22 may instruct beam conditioning assembly 165 toshape a first beam 130 as a collimated beam having a small spatialextent (i.e., spot size). Processor 22 may use this beam configurationfor measuring features disposed on a small sized test pad, as is thecase of logic applications in which metrology is performed on teststructures laid out in the scribe line between adjacent dies of wafer190.

In another example, wafer 190 may comprise a memory device (e.g., DRAM,NAND flash) having large arrays of repeating features (e.g., in thememory blocks), or a logic device having memory sections. In someembodiments, processor 22 may apply to a selected memory block of thedie, a second beam 130 having a larger spot size and higher intensitycompared to first beam 130. Processor 22 may exchange the mirror 122 tofocus beam 130 on the active surface of detector 240 so as to increasethe resolution of the respective SAXS system (e.g., system 10, 30, or 40described above).

In some embodiments, system 10 comprises a beam limiter, also referredto herein as a slit assembly 140, which comprises one or more slitsand/or movable blades described in detail in FIGS. 5 and 6 below. Slitassembly 140 is configured to control and/or refine the position and/orspot size and/or shape and/or convergence or divergence angle ofincident beam 130 on surface 192 of wafer 190.

In some embodiments, system 10 comprises a motorized rotation stage (notshown) having a rotation axis about the y-axis and centered at surface191. In some embodiments, source 100, beam conditioning assembly 165,and one or more of slit assemblies 110 and 140 are mounted on therotation stage, which is controlled by a motion controller and/or byprocessor 22.

In some embodiments, processor 22 may adjust or calibrate the anglebetween incident beam 130 and a normal to surface 192 of wafer 190, soas to improve the measurement conditions of system 10.

In some embodiments, system 10 comprises a chuck 200 having wafer 190mounted thereon. Chuck 200 is configured to mechanically support wafer190 and to allow directing beam 130 to most of the area (e.g.,excluding, at least some part of, the bevel of wafer 190 as shown inFIG. 1), or over the entire area of surface 192.

In some embodiments, chuck 200 may comprise a ring-shaped wafer support,but additionally or alternatively, chuck 200 may comprise any othersuitable design, such as a three-point kinematic mount.

In some embodiments, system 10 comprises a mount, for example, amotorized xyzχωφ-stage, referred to herein as “a stage 210,” havingchuck 200 mounted thereon. Stage 210 is controlled by processor 22 in axyz coordinate system of system 10, and is designed as an open frame(i.e., having no material in the center) so as to allow incident beam130 to directly impinge on surface 192 of wafer 190.

In some embodiments, stage 210 is configured to move wafer 190 relativeto beam 130 in x and y directions, so as to set a desired spatialposition of wafer 190 relative to incident beam 130. Stage 210 isfurther configured to move wafer 190 along z-axis so as to improve thefocus of beam 130 at the desired position on surface 192, or at anyother suitable position on wafer 190. Stage 210 is further configured toapply rotations χ and/or ω about respective x-axis and y-axis parallelto surface 192 of wafer 190, and to apply azimuthal rotation φ aboutz-axis perpendicular to surface 192 of wafer 190.

In some embodiments, processor 22 is configured to select a predefinedazimuth φ so to align beam 130 with selected features in the structuresto be measured. For example, processor 22 may selected a first azimuthφ1 (not shown) so to align beam 130 relative to line structures arrangedin a one-dimensional (1D) on wafer 190. Moreover, processor 22 mayselect a second azimuth φ2 (not shown) so to align beam 130 relative toa pattern or arrays of holes or vias arranged in a two-dimensional (2D)pattern, such as rectangular or hexagonal lattice, on wafer 190.

In alternative embodiments, wafer 190 is mounted on a suitablestationary fixture (instead of stage 210), such that processor 22 canmove source 100, and the aforementioned assemblies (e.g., slit assembly110, and assemblies 165 and 140), so that the X-ray beam is directed toany one or more desired positions of wafer 190. In other embodiments,system 10 may comprise any other suitable set of mounts, such as a setof stages (e.g., a χωφ-stage for wafer 190, and a xyz-stage for theassemblies described above) and processor 22 is configured to movesurfaces 191 and 192 relative to beam 130 by controlling the set ofstages.

In some embodiments, incident beam 130 impinges on surface 192, passesthrough wafer 190 and is scattered from the aforementioned HAR featuresformed in surface 191 of wafer 190. In an alternative configuration ofwafer 190, surface 192 may comprise HAR features, in addition to orinstead of the HAR features patterned in surface 191, as describedabove. In this wafer configuration, incident beam 130 may also bescattered from the HAR features patterned on surface 192. In someembodiments, detector 240 of system 10 is configured to detect X-rayphotons scattered from the HAR features of both surfaces 191 and 192, aswill be described in detail below.

In some embodiments, incident beam 130 may impinge, at a point 111,perpendicular to surface 192 of wafer 190, or at any other suitableangle relative to wafer 190. In an embodiment, some of incident beam 130is absorbed as it traverses wafer 190 and a transmitted beam 220 exitssurface 191 of wafer 190 in the same direction of incident beam 130.Additional beams 222, scattered from the aforementioned one or more HARfeatures, exit at different angles to transmitted beam 130 relative tosurface 191 of wafer 22.

In some embodiments, detector 240 is configured to detect X-ray photonsof beams 222 impinging, at one or more regions 226, on a surface 224 ofdetector 240. Detector 240 may comprise any suitable type of one or moredetectors such as, but are not limited to, charge-coupled devices(CCDs), CMOS cameras provided by a number of suppliers, or arraydetectors made from a silicon (Si) or a cadmium telluride (CdTe)detection layer manufactured by DECTRIS Ltd. (Baden, Switzerland)supplying the 1D Mythen detectors and the 2D Pilatus and Eiger series ofdetectors.

In some embodiments, detector 240 may be mounted on a high-precisionmotorized translation and/or rotation stage (not shown), which isconfigured to move and/or rotate detector 240 based on predefined motionprofiles so as to improve the sensing efficiency thereof. Exampleimplementations of the stage and motion control of detector 240 aredescribed in detail in FIG. 10 below.

In some embodiments, the detectors described above are configured todetect X-rays beams scattered from wafer 190, referred to herein asbeams 222, and comprise sensitive elements of sufficiently small size soas to provide the necessary angular resolution for measuring thesmall-angle scattering intensity distribution from the HAR features ofwafer 190.

In some embodiments, system 10 comprises one or more calibration gauges215, used in calibrating and setting-up system 10, so as to accuratelymeasure properties of the aforementioned features patterned in wafer190. At least one of calibration gauges 215 is configured to produceelectrical signals indicative of the height and inclination of a givenposition at wafer 190 relative to a predefined reference, as will bedescribed in detail below. The electrical signals are sent, viainterface 24, to processor 22 for analysis.

In some embodiments, system 10 may comprise two calibration gauges 215.A first calibration gauge 215, facing surface 192 that is typically flatand has no HAR features or other types of pattern, and a secondcalibration gauge 215, facing surface 191 that is typically patternedand may also have the HAR features described above. In the exampleconfiguration of FIG. 1, the second calibration gauge is optional andtherefore is shown as a dashed rectangle.

In other embodiments, system 10 may comprise any other suitableconfiguration of calibration gauges 215, for example, only the secondcalibration gauge facing surface 191, or having the aforementioned firstand second calibration gauges 215 facing surfaces 192 and 191,respectively.

In some cases, calibration gauge 215 may respond differently to theheight and inclination of a patterned surface (e.g., on surface 191) anda flat surface (e.g., non-patterned or blanket surface 192) of wafer190, and therefore, may require a calibration step before so as toimprove the accuracy of the height and inclination measurements.

In some embodiments, processor 22 may receive from the aforementionedsecond calibration gauge 215, signals indicative of the height andinclination of surface 191, which is patterned. The pattern may affect(e.g., induce shift in) the measurements carried out by the secondcalibration gauge. In these embodiments, processor 22 is configured toadjust or calibrate the angle between incident beam 130 and a normal tosurface 192 of wafer 190, so as to compensate for the pattern inducedshift, and therefore, to improve the quality of measurements carried outby system 10.

Note that when calibration gauge 215 measures the height and inclinationof surface 192, or of any other non-patterned surface, there istypically no shift in the measurements.

In some embodiments, calibration gauge 215, also referred to herein asan optical gauge, may comprise a light source and a sensor (not shown),or any other suitable configuration. Calibration gauge 215 is configuredto measure, at selected coordinates of the x and y axes, the localheight (e.g., distance along z-axis) and inclination of surface 192(e.g., relative to an x-y plane of the xyz coordinate system). In theseembodiments, the light source and the sensor are configured to operatein any suitable wavelength, e.g., visible, infrared (IR), or ultraviolet(UV), but typically not in the X-ray range.

In some embodiments, based on the electrical signals received fromcalibration gauges 215, processor 22 is configured to calculate anddisplay on the display of system 10, a 3D map indicative of the heightand inclination of surfaces 191 and 192, or any other selected plane ofwafer 10, relative to any suitable reference, such as the x-y plane ofthe xyz coordinate system. Processor 22 may calculate the 3D map basedon locations measured on surface 192, and additional locationscalculated between the measured locations, for example, by interpolatingthe height and inclination between two or more of the measuredlocations.

In some embodiments, processor 22 is further configured to determine theone or more starting positions for any X-ray-based alignment procedures.The alignments procedures are used to determine zero angles, referred toherein as ω0 and χ0, of beam 130 relative to one or more scatteringstructures in question by system 10.

In some embodiments, by independently measuring the orientation of (a)surfaces 191 and 192, and (b) scattering features in question (e.g., HARstructures) of wafer 190, relative to incident beam 130, processor 22 isconfigured to calculate the orientation of the scattering featuresrelative to surface 191 of wafer 190. This calculated orientation isparticularly important for measuring HAR structure, such as channelholes of 3D NAND flash memory.

In some embodiments, wafer 190 is typically grown on a crystal havingregular arrangement of the atoms comprising the crystal. Subsequently,wafer 190 is sliced from the crystal, such that the surface is alignedin one of several relative directions, referred to herein as the waferorientation. This is also referred to as the growth plane of thecrystalline silicon. The orientation is important for the electricalproperties of wafer 190. The different planes have differentarrangements of atoms and lattices, which affects the way the electricalcurrent flows in circuit produced in the wafer. The orientations ofsilicon wafers are typically classified using Miller indices, such as(100), (111), (001) and (110).

In some embodiments, system 10 may comprise an integrated opticalmicroscope 50, which may be used for navigation and pattern recognition,and in various other applications, such as optical inspection and/ormetrology, and/or for reviewing pattern and other features on wafer 190.

In some embodiments, optical microscope 50 is electrically coupled tocomputer 20 and is configured to produce signals indicative of thepattern in question, so that processor 22 could perform the patternrecognition or any other of the aforementioned applications.

Additionally or alternatively, system 10 may comprise other suitabletypes of integrated sensors (not shown) configured to provide system 10with complementary metrology or inspection capabilities.

In some embodiments, system 10 comprises one or more X-ray diffraction(XRD) detectors, such as XRD detectors 54 and 56, which are configuredto detect X-ray photons diffracted from planes substantiallyperpendicular to surfaces 191 and 192 of wafer 190.

Reference is now made to an inset 52, which is a top view of system 10.In some embodiments, XRD detectors 54 and 56 are arranged so as toproduce diffraction signal that may be used, as will be described below,for wafer alignment based on X-ray photons diffracted from some planesof the crystal lattice. Signals received from at least one of XRDdetectors 54 and 56 may also be used for other application.

The configuration of XRD detectors 54 and 56, optical microscope 50 andcalibration gauge 215 (optional) as shown in inset 52, is simplified forthe sake of conceptual clarity and is provided by way of example. Inother embodiments, system 10 may comprise any other suitableconfiguration and arrangement of sensors, detectors, microscopes andother suitable components and subsystems.

Reference is now made back to the side view of FIG. 1. In someembodiments, processor 22 may receive from XRD detectors 54 and 56signals indicative of intensity of Laue diffraction from planessubstantially perpendicular to surfaces 191 and 192 of wafer 190. Forexample, crystallographic plane (555) is perpendicular to the surface ofa silicon wafer having a Miller index (001), referred to herein as Si(001). Additionally or alternatively, processor 22 may receive from atleast one of detectors 54, 56 and 240 signals indicative of theintensity of a first portion of beam 222 diffracted from any otherlattice plane of wafer 240. These signals are also referred to herein asdiffraction signals.

In some embodiments, processor 22 is configured to use X-rays diffractedfrom crystal planes substantially normal to surface 191 and sensed byXRD detectors 54 and 56, so as to determine the orientation of theincident beam and/or the direct beam, relative to the lattice planes ofa single-crystal wafer.

In other embodiments, detector 240 is further configured to sense theX-ray photons diffracted from the aforementioned Laue diffraction, andto produce signals indicative of the intensity of the sensed X-rayphotons.

In some embodiments, processor 22 may receive from detector 240 signalsindicative of the intensity of a portion of beam 222 transmitted throughsurface 192 and scattered from the HAR features of surface 191, alsoreferred to herein as scattered signals.

In alternative embodiments, calibration gauge 215 may comprise one ormore X-ray detectors, positioned to measure the Laue diffraction fromplanes substantially perpendicular to surfaces 191 and 192 of wafer 190,and to produce signals indicative of the intensity of the measured Lauediffraction, referred to herein as alternative diffraction signals.

In some embodiments, based on one or more of the diffraction signalsdescribed above, processor 22 is configured to instruct stage 210 toapply ω and χ rotations to wafer 190. Processor 22 may use a position ofwafer 190 corresponding to a maximal intensity of the diffracted X-raydetected by detector 240, for establishing the inclination angles ofbeam 130 relative to the crystal lattice in wafer 190.

In these embodiments, processor 22 is configured to establish theinclination angle between the crystal lattice plane and the surface ofwafer 190, by using measurements at two or more azimuths that satisfythe diffraction condition. Moreover, processor 22 may apply to beam 130X-ray diffraction (XRD) methods, for determining the orientation ofsurfaces 191 and 192, as a calibration technique for non-X-ray basedgauges. For example, calibration may be performed by measuring areference wafer, or any suitable reference structure mounted on acarrier wafer or on the tool, with known inclination angles between thecrystal lattice and surfaces 191 and 192.

In these embodiments, detector 240 may comprise various suitable typesof detection elements, such as but not limited to, (a) arrays of 1Ddiodes made from silicon, germanium or CdTe or other suitable materials,and (b) 2D X-ray direct or indirect detection cameras that are based onCCD, CMOS sensors, PIN diodes, or hybrid pixel detector technologies.

In alternative embodiments, system 10 may comprise, in addition tocalibration gauge 215, an energy dispersive X-ray (EDX) detectorassembly (not shown). The EDX detector assembly comprises asilicon-based or a germanium-based solid-state EDX detector, and anelectronic analyzer having a single-channel or multiple channels. TheEDX detector assembly is configured to measure X-ray fluorescenceemitted, for example, from point 111 of wafer 190, or from a predefinedlocation of a reference wafer used for calibrating system 10, and toproduce an electrical signal indicative of the intensity of X-rayfluorescence measured at point 11.

Based on the electrical signal, processor 22 is configured to determinea first position of point 111 and an offset between the first positionand a second position acquired at the same time, by calibration gauge215.

In some embodiments, X-ray source 100 and at least some of the x-rayoptics between source 100 and wafer 190, are mounted on a first stage,wafer 190 is mounted on a second stage (e.g., stage 210) and at leastone of optical microscope 50 and optical gauges 215 is mounted on athird stage. By comparing between the XRF-based and optical-basedsignals, processor 22 is configured to identify spatial offset, forexample, between an optical pattern recognition camera of opticalmicroscope 50, and X-ray beam 130, and to identify any misalignmentbetween the aforementioned stages of system 10.

In some embodiments, processor 22 is configured to estimate, based onthe received electrical signals, motion errors in stage 210, such asleadscrew errors and non-orthogonality between the x-axis and y-axis ofstage 210. Furthermore, based on the X-ray fluorescence signals,processor 22 is configured to calibrate stage 210, which calibration isalso referred to herein as stage mapping, by estimating offsets betweenone or more points in the coordinate system of system 10, and the actualpositions of the respective points on stage 210.

In some embodiments, system 10 may comprise, in addition to or insteadof the EDX assembly described above, a calibration scheme based onattenuation of the X-ray beam that passes through a suitable referencewafer (not shown), also referred to herein as a direct beam. Thesuitable reference wafer may comprise patterned features adapted toattenuate the direct beam intensity by several tens of percent, so thatdetector 240 could sense photons of the direct beam without beingaffected (e.g., saturated). In an exemplary embodiment, the referencewafer may comprise various patterns having any suitable thickness, e.g.,about 50 μm, of various suitable elements or alloys, such as but notlimited to tungsten (W), tantalum (Ta), gold (Au) or silver (Ag).

In some embodiments, processor 22 may use calibration gauge 215 foraligning between beam 130 and wafer 190 during measurements ofstructures on product wafers, such as wafer 190, or for calibratingsystem 10, e.g., after performing maintenance operations so as toprepare system 10 for use in production.

In alternative embodiments described above, system 10 may comprise atleast one calibration gauge 215 mounted at the opposite side of wafer190 so as to measure the inclination of wafer 190 based on signalssensed from surface 191. In an embodiment, processor 22 is configured tocalibrate an offset between the inclination angles measured on a blanketand the patterned area of a wafer.

In this embodiment, processor 22 positions calibration gauge 215 todirect the optical beam on a first point located adjacent to the edge ofsurface 191, which is typically blanket (i.e., without pattern), andmeasures the inclination of the wafer in x and y axes. Subsequently,processor 22 positions calibration gauge 215 to direct the optical beamat a second point on a pattern in the closest proximity (e.g., 10 mm-20mm) to the first point, and measures the inclination of the wafer in xand y axes.

In some embodiments, based on the inclination measurements at the firstand second points, processor 22 calculates the offset between theblanket and patterned surfaces. Note that the wafers are typicallyrigid, such that the actual inclination angle is not changing within adistance of 10 mm or 20 mm. The offset may be used as a calibrationfactor between inclination measurements on blanket and patternedsurfaces of wafer 190 or any other type of measured wafer. In someembodiments, processor 22 may set the spot size of the optical beam tobe sufficiently small to illuminate only the blanket surface near thewafer edge, but sufficiently large to average the inclinationmeasurement over various features of the pattern.

In some embodiments, wafer 190 comprises a single-crystalline material,and at least one of XRD detectors 54 and 56 is configured to measure thediffraction of beam 220 from a lattice plane of the single-crystalmaterial. In some embodiments, in response to the measured diffraction,processor 22 is configured to calibrate suitable parameters (e.g.,orientation) of calibration gauge 215 with respect to the lattice plane.

The particular configuration of calibration gauge 215 is shown in FIG. 1schematically, so as to demonstrate calibration techniques for improvingthe measurements of features, such as HAR structures, of wafer 190,carried out by system 10. Embodiments of the present invention, however,are by no means limited to this specific sort of example configuration,and the principles of calibration gauge 215 described above, may beimplemented using any suitable configuration.

In an embodiment, system 10 comprise a beam-blocking assembly, referredto herein as a beam blocker 230, made from an X-ray opaque orpartially-opaque material.

Beam blocker 230 is mounted in system 10 between wafer 190 and detector240, and is configured to occlude at least part of beam 220 fromirradiating detector 240. In some cases at least part of incident beam130 may be directly transmitted through wafer 190.

In some embodiments, beam blocker 230 may be positioned so as topartially block the directly-transmitted incident beam over an angularrange comparable to the spatial extent of incident beam 130.

Example implementations of beam blockers are depicted in detail in FIGS.7A and 7B below.

In some embodiments, the opaqueness level and shape of beam blocker 230affect the signals produced by detector 240, as depicted in FIGS. 8A,8B, 9A and 9B below.

In some embodiments, the detector assembly may comprise a singledetector, or an array of detectors arranged around regions 226. The beamdetectors may have a 2D configuration (i.e, an area detector), or a 1Dconfiguration (i.e, a linear detector), and are capable of countingX-ray photons. Detector 240 may be flat, or may have any suitable shapesuch as an arc angled toward beams 222 and 220. Responsively to thecaptured photons, 240 is configured to generate electrical signals,which are conveyed, via interface 24, to processor 22. One exampleimplementation of detector 240 is depicted in detail in FIG. 10 below.

In some embodiments, system 10 comprises a vacuum chamber 280, mountedbetween wafer 190 and detector 240 and configured to reduce undesiredscattering of beam 220 from air. In some embodiments, vacuum chamber 280comprises a metal tube with windows transparent to X-ray at each end, sothat beams 220 and 222 can pass between wafer 190 and detector 240.

In some embodiments, system 10 comprises a suitable vacuum pump, such asa roughing pump controlled by processor 22, so as to control the vacuumlevel in vacuum chamber 280, thereby to improve signal-to-background(SBR) ratio of X-ray photons impinging on the active surface of detector240.

In some embodiments, system 10 is configured to measure structural(e.g., dimensions and shape) as well as morphological parameters on theaforementioned features of wafer 190. For example, based on theelectrical signals received from detector 240, processor 22 isconfigured to measure a large variety of parameters, such as but notlimited to height, depth, width and sidewall angle of the patternedstructure, and thickness and density of films at any location acrosswafer 190.

In some embodiments, processor 22 comprises a model-based software foranalyzing the electrical signals received from detector 240. Processor22 uses a single structural model so as to simulate the X-ray scatteringfor all incidence angles having a common intensity normalization factor.Subsequently, processor 22 compares the correlation between the measuredand simulated intensity distributions, e.g., based on a numericalanalysis of a goodness-of-fit (GOF) parameter.

In some embodiments, processor 22 is configured to iteratively adjustthe parameters of the model, for example by using an algorithm such asDifferential Evolution (DE), so as to minimize the GOF parameter and toobtain the best-fit model parameters.

In some embodiments, processor 22 may reduce the correlation betweenmodel parameters by introducing into the model parameter values measuredby complementary techniques, for example the width at the upper layer ofa feature in question measured by a critical dimension scanning electronmicroscope (CD-SEM).

In some embodiments, system 10 may comprise one or more calibrationtargets having arrays of periodic features externally characterizedusing any suitable reference technique other than SAXS, e.g., atomicforce microscope (AFM). Processor 22 may use the calibration targets asa reference for calibrating the aforementioned assemblies of system 10and for alignment between (a) beam 130 and wafer 190, and (b) betweenbeam 222 and detector 240.

In some embodiments, based on the SAXS configuration and the softwarealgorithms described above, system 10 is configured to detect disorderparameters in the features in question across wafer 190. For example,horizontal and vertical roughness of the sidewalls and pitch variation,such as a pitch-walking error that may appear, for example, inmulti-patterning lithography processes or tilting and twisting of thechannel holes due to the etch process in 3D NAND memory.

In the context of the present disclosure and in the claims, the terms“small angle” and “small-angle” of the SAXS refer to an angle smallerthan 10 degrees relative to the direct beam.

The configuration of system 10 is shown by way of example, in order toillustrate certain problems that are addressed by embodiments of thepresent disclosure and to demonstrate the application of theseembodiments in enhancing the performance of such a system. Embodimentsof the present invention, however, are by no means limited to thisspecific sort of example system, and the principles described herein maysimilarly be applied to other sorts of X-ray systems used for measuringfeatures in any suitable type of electronic devices.

FIG. 2 is a schematic illustration of a SAXS system 30, in accordancewith another embodiment of the present invention. In some embodiments,the configuration of SAXS system 30, also refers to herein as “system30” for brevity, is similar to the configuration of system 10 with wafer190 tilted, also referred to herein as rotated, at any suitable angle(e.g., 45 degrees) relative to incident beam 130.

In some embodiments, processor 22 is configured to instruct stage 210 totilt wafer 190 about a tilt axis, such as azimuthal rotation ω abouty-axis, in a plane of wafer 190, and to orient at least one of theaforementioned slit assemblies parallel to the tilt axis.

In some embodiments, system 30 is configured to measure structures ofwafer 190 having a low aspect ratio (e.g., height over width ratiosmaller than ten). As described above, processor 22 is configured torotate wafer 190 relative to incident beam 130, or alternatively, torotate incident beam 130 relative to wafer 190. Processor 22 isconfigured to carry out the rotation over a range of several tens ofdegrees around y-axis, referred to herein as ω rotation.

In some embodiments, the range of rotation angles may be symmetric, forexample ±50 degrees relative to the surface of wafer 190 shown, forexample, in FIG. 1 above.

In alternative embodiments, processor 22 may carry out asymmetricrotation (e.g., −10 degrees to +60 degrees), for example by instructingstage 210 to rotate wafer 190 to a desired angle within theaforementioned range.

In some embodiments, processor 22 is configured to measure a profile ofa structures in more than one plane, for example, by rotating theazimuth of wafer 190 relative to beam 130. In the context of the presentdisclosure and in the claims, the term “profile” refers to a shape of asingle sidewall of a measured feature, or a change of width between twoadjacent sidewalls along the depth or height thereof or shift of thecenter of the hole as a function of depth. Additional asymmetry of theholes such as elliptical rather than circular cross section will usuallyrequire measurements at different azimuth and chi axes.

For example, processor 22 may measure the profile of a feature in aselected xy-plane using a series of intensity measurements carried outat different azimuthal angles. In some embodiments, processor 22 mayimplement this technique for measuring the diameter of a channel hole ina 3D NAND memory device, or the width of a via and/or metal line oflocal interconnect structures of a logic device.

In an embodiment, beam blocker 230 is positioned in close proximity todetector 240. In another embodiment, beam blocker 230 may be positionedin close proximity to wafer 190.

FIG. 3 is a schematic illustration of a SAXS system 40, in accordancewith another embodiment of the present invention. In some embodiments,the configuration of SAXS system 40, also refers to herein as “system40” for brevity, is similar to the configuration of system 10 with beamblocker 230 positioned in close proximity to wafer 190.

In some embodiments, processor 22 is configured to control the positionof beam blocker 230 at any suitable position along the path of beam 220,so as to reduce the level of undesired background and stray scatteringsensed by detector 240.

In some embodiments, processor 22 may set the position of beam blocker230 at one or more predefined mounting locations along the path of beam220. Additionally or alternatively, processor 22 may adjust the positionof beam blocker 230 by controlling a motorized stage (not shown)configured to move and hold beam blocker 230 at any suitable positionbetween wafer 190 and detector 240.

The structure of beam blocker 230 and related assemblies, such as theaforementioned stage, are described in detail, for example, in FIG. 7Abelow. Moreover, embodiments related to the functionality andapplications of beam blocker 230 in measuring features in question ofwafer 190 are described in detail in FIGS. 8B and 9B below.

The configurations of systems 10, 30 and 40 are provided by way ofexample. Embodiments of the present invention, however, are by no meanslimited to this specific sort of example systems, and the principlesdescribed herein may similarly be applied to other sorts of metrologysystems, such as but not limited to, reflection-based X-ray metrologysystems having both the X-ray source and detector assemblies located atthe same side of the wafer.

FIG. 4 is a schematic illustration of beam conditioning assembly 165, inaccordance with an embodiment of the present invention. Beamconditioning assembly 165 may be used in any of systems 10, 30 and 40described above, or in any other suitable configuration of a metrologysystem that applies X-ray beams for measuring features produced in wafer190 or any other type of wafer.

In some embodiments, beam conditioning assembly 165 comprises multiplesets of slit assemblies, referred to herein as assemblies 110, 300 and320. Note that as shown in FIGS. 1-3, assembly 110 may be external tobeam conditioning assembly 165, or incorporated therein as shown in FIG.4. Similarly, assembly 320 may be part of, or external to, beamconditioning assembly 165.

As described in FIG. 1 above, the slit assemblies of beam conditioningassembly 165 are configured to block undesired scattered X-ray radiationdeflected from the designed optical path of beam 130, and/or to adjustthe divergence, intensity and spot-size of beam 130.

In some embodiments, beam conditioning assembly 165 comprises mirror120, which is configured to shape the optical properties of beam 130after the beam passes through assembly 110, as described in FIG. 1above.

In some embodiments, mirror 120 comprises a curved substrate 122 coatedwith multiple layers 124, for example, alternating thin (e.g., an orderof one micron) layers of a heavy element, such as W, Mo or nickel (Ni),with a light element, such as carbon or silicon. Such mirrors for X-rayoptics are provided by several suppliers, such as Incoatec GmbH(Hamburg, Germany), AXO DRESDEN GmbH (Dresden, Germany) or Xenocs(Sassenage, France). In some embodiments, the configuration of mirror120 is adapted to provide a collimated beam in two directions (x,y). Inother embodiments, mirror 120 is configured to collimate beam 130 in onedirection (e.g., x-direction) and to focus beam 130 in an orthogonaldirection (e.g., y-direction).

In some embodiments, mirror 120 is configured to focus beam 130 onsurface 191, so as to obtain the smallest spot-size. In otherembodiments, focusing the X-ray beam on detector 240 may provide system10 with improved angular resolution of the X-ray beam sensed by detector240, e.g., in imaging of the HAR structures.

In case of a 2D collimated beam, beam conditioning assembly 165 maycomprise two optics, e.g., two mirrors 120, facing one another so as toincrease the solid angle (i.e., a two-dimensional angle) collected fromsource 100 and to increase the X-ray flux of beam 130.

In some embodiments, beam conditioning assembly 165 may comprise anysuitable configuration of multiple multilayered mirrors, such as mirror120, mounted on one or more motorized actuators controlled by processor22. Processor 22 may arrange the configuration of each mirror 120 ofbeam conditioning assembly 165, so as obtain the most suitablemeasurement conditions by adjusting the optical properties of beam 130.

In some embodiments, beam conditioning assembly 165 comprises a crystal310, made from a single crystal of germanium (Ge) or any other suitablematerial. Crystal 310 has a v-shaped channel 312 comprising an entranceaperture 316, an exit aperture 318, and opposing internal faces 314 and315 arranged so that channel 312 tapers from entrance aperture 316 toexit aperture 318, which is smaller than aperture 316.

In some embodiments, beam 130 passes through slit assembly 110 intomirror 120 and subsequently passes through slits assembly 300 andentrance aperture 316. Subsequently, beam 130 impinges on internal face314 and thereafter on internal face 316 and exits crystal 310 throughexit aperture 318.

In some embodiments, beam conditioning assembly 165 serves as adispersing element, and additionally as beam compressing opticconfigured to reduce the spot size of beam 130 after exiting slitassembly 320 of assembly 165. The configuration of beam conditioningassembly 165 enables the beam compressing, and yet, reduces the loss offlux compared to alternative techniques, such as a crystal with achannel having parallel faces or using one or more slits having one ormore narrow apertures.

In the example configuration of FIG. 4, slit assemblies 110, 300 and 320are mounted before and after mirror 120 and crystal 310 so as to improvethe shaping of beam 130 along the optical path described above. In otherembodiments, beam conditioning assembly 165 may comprise any othersuitable configuration of slit assemblies interposing between source 100and mirror 120, and/or between mirror 120 and crystal 310, and/orbetween crystal 310 and slit assembly 140 or any other component orassembly of any of systems 10, 30 and 40. For example, slit assembly 320may be removed from the configuration of assembly 165 and may beexcluded from the configuration of any of systems 10, 30 and 40.

FIG. 5 is a schematic illustration of slit assembly 140, in accordancewith an embodiment of the present invention. As shown in FIGS. 1-3, slitassembly 140, also referred to herein as a beam limiter, is positionedbetween source 100 and surface 192 of wafer 190, so as to intercept beam130.

In some embodiments, slit assembly 140 comprises two or more movableplates 520 positioned along a translation axis 522 in a predefineddistance from one another so as to define a slit 512. The distancebetween plates 520 may be controllable by processor 22, for exampleusing one or more actuators (not shown) for moving one or more plates520 along translation axis 522. Alternatively, the distance betweenplates 520 may be constant, e.g., by not moving plates 520 relative toone another, or by selecting a suitable type of slit 512 having staticplates positioned at a desired distance from one another.

In some embodiments, slit assembly 140 comprises two or more movableblades 510A and 510B, which are not parallel to one another and haverespective edges 514A and 514B positioned in close proximity to oneanother, so as to define a micro-slit 515.

In some embodiments, micro-slit 515 is configured to block part of beam130 that impinges on blades 510A and 510B without producing scatteredbeams, thus blades 510A and 510B are also referred to herein as“anti-scatter blades.” In some embodiments, blades 510A and 510B aremade from single-crystal materials such as tantalum (Ta), Ge,indium-phosphide (InP), or from polycrystalline materials such astungsten-carbide, and have a thickness of about 1 mm or any othersuitable thickness.

In the context of the present disclosure, and in the claims, the terms“single-crystal” and “mono-crystal” are used interchangeably and referto materials having a structure made from one crystal.

In some embodiments, slit assembly 140 comprises actuators 500A and500B, configured to move respective blades 510A and 510B alongrespective translation axes 516A and 516B, so as to adjust the width ofmicro-slit 515. In an embodiment, at least one of translation axes 516Aand 516B is substantially orthogonal to translation axis 522 in the x-yplane.

In some embodiments, actuators 500A and 500B comprise one or morepiezoelectric linear motors, for example the Piezo LEGS Linear 6G seriesprovided by PiezoMotor (Uppsala, Sweden) or similar products from othervendors such as Physik Instrumente (Karlsruhe, Germany). These motorscan be supplied with integrated high-resolution position sensors.

In some embodiments, processor 22 is configured to position slitassembly 140 in any suitable proximity to surface 192 of wafer 190. Thedesign of micro-slit 515 allows processor 22 to position slit assembly140 such that at least one of edges 514A and 514B is positioned at adistance smaller than ten millimeters (10 mm) from surface 192. In otherembodiments, processor 22 may position micro-slit 515 at any selecteddistance, e.g., between 100 mm and a few millimeters, from surface 192.

In some embodiments, the configuration of micro-slit 515 allowsprocessor 22 to position slit assembly 140 in close proximity (e.g.,down to a few millimeters) to surface 192 even when wafer 190 is tilted,as shown in FIG. 2 above.

In some embodiments, processor 22 is configured to set the distancesbetween (a) micro-slit 515 and surface 192, (b) edges 514A and 514B, and(c) plates 520, so as to obtain the desired optical properties of beam130 before impinging on surface 192 and interacting with the structuresand bulk of wafer 190.

Reference is now made to an inset 502, which is a top view of theinterception between slit assembly 140 and beam 130. In the example ofinset 502, processor 22 is configured to change the spatial shape ofbeam 130 from a round shape of a circle 524, to a rectangular shapeshown by a dashed rectangle 526, by moving (a) blades 510A and 510Balong respective translation axes 516A and 516B, and (b) plates 520along translation axis 522. Note that in this example, only the portionof beam 130 within the area of dashed rectangle 526 impinges on surface192, whereas the remaining part of beam 130, located between the edgesof circle 524 and dashed rectangle 526, is blocked by slit assembly 140.As described above and shown in inset 502, at least one of translationaxes 516A and 516B is orthogonal to translation axis 522.

The configuration of slit assembly 140 is simplified for the sake ofconceptual clarity and is provided by way of example. In otherembodiments, slit assembly 140 may comprise more than two blades 510Aand 510B, and/or more than two plates 520. Moreover, the edge of plates520 and/or edges 514A and 514B may have any suitable shape, for example,both plates 520 and edges 514A and 514B may have an arc intruding intothe area of the respective plates 520 and blades 510A and 510B, so as toform a round shape, rather than the aforementioned rectangular shape, ofbeam 130 exiting from slit assembly 140.

In other embodiments, translation axes 516A and 516B may be parallel ornot-parallel to one another, and at least one of translation axes 516Aand 516B may not be orthogonal to translation axis 522.

FIG. 6 is a schematic illustration of a slit assembly 150, in accordancewith another embodiment of the present invention. Slit assembly 150 mayreplace, for example, slit assembly 140 shown in FIGS. 1-3.

In some embodiments, slit assembly 150 comprises a 3-pinhole collimationsystem, also referred to herein as apertures 604, 606 and 608, arrangedalong a translation axis 610 of a movable blade 550.

In some embodiments, slit assembly 150 comprises an actuator 600,configured to move blade 550 along translation axis 610.

Reference is now made to an inset 602, which is a top view of aninterception between beam 130 and blade 550.

In some embodiments, each aperture 604, 606 and 608 comprises a fixedsize apertures, such as a SCATEX scatterless pinhole produced byIncoatec GmbH (Hamburg, Germany). In the example of blade 550, apertures604, 606 and 608 have a round shape and each aperture has a differentdiameter, e.g., between about 20 μm and 500 μm.

In some embodiments, blade 550, which serves as a frame of thescatterless pinholes, is made from Ge for X-ray beams having low energyof photons, or from Ta for beams having photons with higher energies.

In some embodiments, the configuration of apertures 604, 606 and 608 isadapted to reduce undesired parasitic scattering typically occurs whenan X-ray beam passes through other types of apertures.

In some embodiments, actuator 600 may comprise any suitable type ofmotor coupled to a drive rod 620, which is configured to move blade 55along translation axis 610.

In other embodiments, the configuration of actuator 600 may be similarto the configuration of actuators 500A and 500B described in FIG. 5above.

In some embodiments, processor 22 is configured to determine the opticalproperties of beam 130 by instructing actuator 600 to position aselected aperture of blade 550 to intercept beam 130. In the example ofFIG. 6, actuator 600 positions aperture 606 so that beam 130 passestherethrough, and the portion of beam 130 that exceeds the area withinaperture 606 will be blocked.

FIG. 7A is a schematic illustration of beam blocker 230, in accordancewith an embodiment of the present invention. In some cases, at leastpart of incident beam 130 that impinges on surface 192 is transmitteddirectly through wafer 190 and exits from surface 191, as part of beam220, without being scattered. The directly transmitted part of beam 220is referred to herein as a “direct beam.”

In some embodiments, beam blocker 230 is positioned so as to attenuatethe X-ray radiation of the direct beam, typically at the center of beam220. This attenuation is necessary, for example, to prevent damage todetector 240 and/or to prevent the detector from saturation and fromoperating in a non-linear region. On the other hand, too largeattenuation would eliminate the detection of essential signals that maybe used by processor 22 for tracking the angular position and intensityof the center of beam 220. Thus, the attenuation of beam blocker 230 istypically selected so that the intensity of the transmitted beam isattenuated to a few hundreds or thousands of photons per second atdetector 240.

In some embodiments, beam blocker 230 comprises one or more beamblocking elements, such as a beam stopper 232, typically having anellipsoidal-shape or any other suitable shape. In some embodiments, beamstopper 232 is made from an X-ray partially-opaque material, alsoreferred to as high-Z materials, typically comprising metal elements,such as tantalum or tungsten, and/or any suitable metal alloys.

As described above, the attenuation of beam stopper 232 is selected toenable reliable measurement of the angular position and intensity ofbeam 220, and at the same time prevent damage and non-linear distortionin the sensing of detector 240.

In some embodiments, beam stopper 232 is further configured to minimizebackground intensity from sources such as scattering with air orfluorescence and other scattering from the electronics behind the activeregion or surface of detector 240. Note that the active region ofdetector 240 may be partially illuminated due to the limited thicknessor low absorption of the detector material, e.g. 450 μm of silicon, withhigh energy X-rays having an energy of 10 keV or higher.

In some embodiments, beam stopper 232 has a curvy and/or smooth edge soas to reduce the scattering intensity of the direct beam.

In some embodiments, beam blocker 230 comprises a matrix 236, alsoreferred to herein as a mount. Matrix 236 is made from a block ofmaterial adapted not to scatter X-rays, such as, but not limited to,diamond or polymers such as thin-sheets of biaxially-orientedpolyethylene terephthalate (BoPET) polyester, also referred to herein asMylar™, or poly (4,4′-oxydiphenylene-pyromellitimide) polyimide, alsoreferred to herein as Kapton®.

In some embodiments, beam stopper 232 is mounted in a recess (not shown)formed in a matrix 236, and is mechanically supported by the matrixmaterial without using an adhesive that may scatter X-rays, andtherefore, may increase the level background signals to themeasurements. Since adhesives may degrade under X-ray irradiation withtime, absorbing features can be fabricated using techniques used forelectronics manufacturing such as depositing thin adhesion and seedlayers with appropriate metalizeation and then electroplating a thickX-ray absorbing material such as gold (Au), or by using of additiveprinting techniques using inks incorporating a high concentration ofmetallic nanoparticles followed by an annealing process.

In other embodiments, beam stopper 232 may be coupled to matrix 236using any other suitable technique, such as an adhesive that does notscatter X-rays. Note that beam stopper 232 is adapted to attenuate thedirect beam, such that the surrounding scattered beam, shown in FIG. 1as beam 222, is not attenuated since the support structure istransparent to the scattered X-rays of beam 222.

In some embodiments, the material of beam stopper 232 allows sufficientintensity of the direct beam to be partially transmitted, so thatprocessor 22 may determine the intensity and position of the direct beamsensed by detector 240 without moving beam stopper 232 away from thedirect beam.

In some embodiments, beam blocker 230 comprises a mount, also referredto herein as a high-precision motorized stage 233, which is controlledby processor 22 and is configured to move along one or more axes. Forexample translation x-axis and y-axis in the configuration of systems 10and 30 shown, respectively, in FIGS. 1 and 2 above.

In some embodiments, matrix 236 is mounted on stage 233 so thatprocessor 22 sets the position of beam stopper 232 relative to thedirect beam transmitted through wafer 190. In other embodiments, stage233 may comprise rotational axes (not shown) so as to improve thealignment of beam stopper 232 with beam 220, and particularly with thedirect beam thereof. In another embodiment, stage 233 is also configuredto move in z-axis so as to enable the configuration of system 40, shownin FIG. 3 above, or to further improve the attenuation level of thedirect beam.

In some cases, the attenuation of the direct beam may be sufficientlyhigh by wafer 190 or by any other element of system 10. Thus, in otherembodiments, processor 22 is configured to move beam blocker 230 awayfrom the path of beam 220. In these embodiments, beam stopper 232 is notintercepting beam 220, so that processor 22 may monitor the intensityand position of the direct X-ray beam, based on the direction andintensity of the direct beam sensed by detector 240.

The configuration of beam blocker 230 is simplified for the sake ofconceptual clarity and is provided by way of example. In otherembodiments, beam blocker 230 may comprise any other suitable componentsand/or assemblies arranged in any other suitable configuration forattenuating the intensity of the direct beam and/or for managing thesensing of one or more beams 222 scattered from wafer 190. For example,the beam-blocker may comprise multiple beam stoppers 232, or two narrowwires whose separation can be adjusted so as to change the effectivewidth of the blocker.

FIG. 7B is a schematic illustration of beam blocker 330, in accordancewith an embodiment of the present invention. Beam blocker may replace,for example, beam blocker 230 of FIG. 1 above. In some embodiments, beamblocker 330 comprises a matrix 333 made from a synthetic diamond, or thematerials of matrix 236 described above, or any other suitable materialadapted not to scatter X-rays of beam 220.

In some embodiments, beam blocker 330 comprises multiple types of beamstoppers, each of which made from a suitable material. For example, agold-based beam stopper having a thickness of about 50 μm, or any othersuitable thickness, or a tungsten-based beam stopper having a typicalthickness between 50 μm and 100 μm, or any other suitable thickness. Thetungsten-based beam stopper may be produced, for example, by lasercutting of a suitable tungsten foil.

In some embodiments, the beam stoppers are coupled to matrix 333, usingany suitable technique, such as recessing the matrix and disposing thebeam stopper into the recessed pattern, or any other suitable method,such as those described in FIG. 7A above. For example, gold or tantalummay be deposited into the recessed pattern or deposited using chemicaland/or physical techniques on the surface of the matrix, and thelaser-cut tungsten pieces, described above, may be attached to therecessed pattern.

In some embodiments, beam blocker 330 comprises multiple geometricshapes and arrangements of beam stoppers. In the example of FIG. 7B,beam blocker 330 comprises five bar-shaped beam stoppers arranged in arow along X-axis, at a 5 mm distance from one another, and having asimilar length (measured along Y-axis) of about 10 mm. The bar-shapedbeam stoppers have different width, e.g., between 0.1 mm and 0.5 mm. Forexample, beam stoppers 332 and 334 have a width (measured along X-axis)of about 0.5 mm and 0.3 mm, respectively, and the bar between beamstoppers 332 and 334 has a width of about 0.4 mm.

In some embodiments, beam blocker 330 comprises five square-shaped beamstoppers having the same arrangement along X-axis (e.g., width anddistance) of the bar-shaped beam stoppers described above. For example,beam stoppers 336 and 338 have widths of 0.4 mm and 0.2 mm,respectively, and the square-shaped beam stopper laid out therebetweenhas a width of 0.3 mm.

In some embodiments, beam blocker 330 may comprise other shapes of beamstoppers such as rectangular, elliptical shapes. Beam blocker 330 maycomprise additional marks to assist with alignment of the blocker, suchas marks 337 and 339

The configuration of beam blocker 330 is provided by way of example. Inother embodiments, beam blocker 330 may comprise any other set of beamstoppers, having any suitable shape and dimensions and arranged in anysuitable layout.

FIG. 8A is a schematic illustration of an image 402 indicative of theintensity of beam 220 sensed by detector 240 in the absence of beamblocker 230, in accordance with another embodiment of the presentinvention. In the example of FIG. 8A, incident beam 130, which iscollimated in both x-axis and y-axis, impinges on wafer 190 comprising ahexagonal array of features, such as HAR capacitors of a DRAM device.

In some embodiments, image 402 comprises a spot 420, indicative of theintensity of the direct beam sensed by detector 240. Image 402 furthercomprises multiple spots 410, indicative of respective beams 222scattered from the hexagonal array of the DRAM device. In someembodiments, the gray level of spots 410 and 420 is indicative of theintensity of beam 220 (e.g., flux of photons and respective energythereof) sensed by detector 240. In the present example, white color isindicative of high intensity, and darker colors indicative of lowerintensities sensed by detector 240.

In some embodiments, image 402 comprises locations 404 positionedbetween spots 410 and spot 420, within region 226 of detector 240, alsoshown in FIG. 1 above. Image 402 further comprises a region 400,referred to herein as a background, located out of region 226 ofdetector 240.

In some embodiments, processor 22 is configured to set the properties ofbeam 130, such that (a) spots 410 have coherent scattering, andtherefore appear bright, (b) locations 404 between spots 410 haveincoherent scattering, and therefore appear darker than spots 410located within a virtual circle 405 surrounding an area in closeproximity to spot 420, and (c) region 400 has no scattering, or ascattering level below a predefined threshold, and therefore appears inblack.

In some embodiments, in the absence of beam blocker 230, the highintensity of the direct beam causes saturation of detector 240 at thearea of spot 420, and therefore, non-linear sensing across region 226.Therefore, spot 420 appears in white color and the area within circle405 appears substantially brighter than the peripheral area of region226.

As described above, due to the coherent scattering, spots 410 appearbrighter than locations 404 within the area of circle 405. However, dueto the increase in the incoherent background from detector 240, spots410 appear darker than locations 404 at the periphery of region 226.Thus, the reliable sensing area of detector 240 is limited to the areawithin circle 405, subject to the limited contrast caused by theincreased background (incoherently X-ray intensity) from detector 240.

FIG. 8B is a schematic illustration of an image 406 indicative of theintensity of beam 220 sensed by detector 240 in the presence of beamblocker 230, in accordance with an embodiment of the present invention.Similar to the example of FIG. 8A, incident beam 130, which iscollimated in both x-axis and y-axis, impinges on wafer 190 comprisingthe hexagonal array HAR capacitors of the aforementioned DRAM device.

In some embodiments, image 406 comprises a spot 430, indicative of theintensity of the direct beam sensed by detector 240. Image 406 furthercomprises multiple spots 440, indicative of respective beams 222scattered from the hexagonal array of the DRAM device.

In some embodiments, beam blocker 230 attenuates the intensity of thedirect beam sensed by detector 240, therefore, spot 430 appears in adark gray color and detector 240 does not introduce a significantbackground intensity shown, for example, in FIG. 8A above.

In some embodiments, the sensed intensity of the coherent scatteringfrom the HAR features appears stronger within circle 405 compared to theperiphery of region 226. Yet, the linear sensing of detector 240 reducesthe intensity detected from locations 404 to the background level ofregion 400. Thus within region 226, the contrast between all spots 440and regions 404 is sufficiently high to conduct measurements at highaccuracy and precision. The term “accuracy” refers to measuring theactual size of the feature in question, and the term “precision” refersto the repeatability of multiple measurements carried out on a givenfeature in question.

In some embodiments, the presence of beam blocker 230 allows processor22 to monitor the partially attenuated direct beam (e.g., during themeasurements of the HAR structures) so as to control parametersindicative of the properties of beams 130 220, such as the incident fluxof both beams 130 and 220 at respective positions on wafer 190 anddetector 240.

FIG. 9A is a schematic illustration of an image 502 indicative of theintensity of beam 220 sensed by detector 240 in the absence of beamblocker 230, in accordance with another embodiment of the presentinvention. In the example of FIG. 9A, incident beam 130, which iscollimated in x-axis and focused on wafer 190 (e.g., on surface 191) iny-axis, impinges on wafer 190 comprising an array of 1D (lines) or longand narrow 2D features, such as lines or trenches in a device ordedicated metrology pad in the scribe-line or elsewhere on die.

In some embodiments, image 502 comprises a spot 526, indicative of theintensity of the direct beam sensed by detector 240. Image 502 furthercomprises multiple features 510, indicative of respective beams 222scattered from the array. In some embodiments, the gray level offeatures 510 and spot 526 is indicative of the intensity of beam 220sensed by detector 240. As described in FIG. 8A above, white color isindicative of high intensity, and darker colors indicative of lowerintensities sensed by detector 240.

In some embodiments, image 502 comprises locations 504 positionedbetween features 510 and spot 526, within region 226 of detector 240.Image 502 further comprises region 400, located out of region 226 ofdetector 240.

In some embodiments, processor 22 is configured to set the properties ofbeam 130, such that features 510 have coherent scattering, locations 504have incoherent scattering, and region 400 has no scattering.

In some embodiments, in the absence of beam blocker 230, the highintensity of the direct beam causes sufficiently high backgroundintensity and loss of contrast across region 226. Therefore, spot 526appears in white color and the area within a virtual rectangle 505appears substantially brighter than the peripheral area of region 226.

As described above, due to the coherent scattering, features 410 appearbrighter than locations 504 within the area of circle 405. However, theincreased background from detector 240, results in loss of contrast atthe periphery of region 226. Thus, the reliable sensing area of detector240 is limited to the area within rectangle 505. Note that in theabsence of beam blocker 230, the shape and size of the reliable sensingarea of detector 240 depends on the type (e.g., geometry) of themeasured features (e.g., round in FIG. 8A and linear in FIG. 9A), theproperties of beam 130, and other parameters of the system, such as thetilt angle of wafer 190 shown, for example, in system 30 of FIG. 2above.

FIG. 9B is a schematic illustration of an image 506 indicative of theintensity of beam 220 sensed by detector 240 in the presence of beamblocker 230, in accordance with an embodiment of the present invention.In some embodiments, processor 22 sets incident beam 130 in a likemanner to the setting described in FIG. 9A above.

Therefore, beam 130, which is collimated in x-axis and focused iny-axis, impinges on wafer 190 comprising the aforementioned layout ofthe lines or trenches.

In some embodiments, image 506 comprises a spot 530, indicative of theintensity of the direct beam sensed by detector 240. Image 506 furthercomprises multiple features 540, indicative of respective beams 222scattered from the array of the NAND flash memory device.

In some embodiments, beam blocker 230 attenuates the intensity of thedirect beam sensed by detector 240, therefore, spot 530 appears in adark gray color and detector 240 is not saturated by excess intensity.

In some embodiments, the sensed intensity of the coherent scatteringfrom the lines or trenches, appears stronger within rectangle 505compared to the periphery of region 226. Yet, the linear sensing ofdetector 240 reduces the intensity detected from locations 504 to thebackground level of region 400. Thus within region 226, the contrastbetween all features 540 and regions 504 is sufficiently high to conductmeasurements at high accuracy and precision.

As described in FIG. 8B above, the presence of beam blocker 230 allowsprocessor 22 to monitor the partially attenuated direct beam so as tocontrol parameters indicative of the properties of beams 130 220.

FIG. 10 is a schematic illustration of a scanning scheme in whichdetector 240 comprising an array of sensors 243 is moved at stepssmaller than the inter-distance of the sensors, for improved angularresolution, in accordance with an embodiment of the present invention.In some embodiments, detector 240 comprises an array of 1D or 2D sensorelements, referred to herein as sensors 243. In the example of FIG. 10detector 240 comprises 2D sensors 243, each of which has a predefinedpitch in x-axis and in y-axis, referred to herein as Px and Py,respectively.

In the context of the present disclosure, and in the claims, the terms“Px” and “width axis” are used interchangeably, and the terms “Py” and“height axis” are also used interchangeably. In some embodiments, eachsensor 243 is configured to produce electrical signals indicative of theintensity of the direct beam and of beam 222 impinging on the activesurface thereof. In some embodiments, processor 22 is configured toproduce an image, referred to herein as a pixel, based on the electricalsignals received from each sensor 243. Thus, the size of each pixel in xand y axes is typically on an order of Px and Py, respectively.

In some embodiments, detector 240 is mounted on a motorized stage 246comprising translation and rotation motors (not shown). In someembodiments, the translation motors are configured to move detector 240in x-axis and y-axis for scanning in the x-y plane, and in z-axis forimproving the focus of beam 222 on the active surface of sensors 243. Insome embodiments, the translation motors are configured to rotatedetector 240, for example about z-axis, for aligning sensors 243 withthe direction of the scattered X-ray photons of beam 222.

In some embodiments, stage 246 comprises high-precision encoders and/orinterferometers (not shown) configured to measure the translation androtation positions of the respective axes of stage 246 at a predefinedfrequency.

In some embodiments, system 10 may comprise a motion control assembly(not shown), which is controlled by processor 22. The motion controlassembly comprises a controller (not shown) configured to determine, foreach motor, a respective motion profile (e.g., speed, acceleration anddeceleration). The motion control assembly further comprises one or moredrivers, which are controlled by the aforementioned controller and areconfigured to drive the motors of stage 246 to move in accordance withthe respective motion profile and based on the current position measuredby the respective encoder or interferometer of each axis.

In other embodiments, processor 22 is further configured to control themotion of stage 246, and may be used for this purpose, in addition to orinstead of the controller.

In some embodiments, stage 246 is configured to move detector 240 alongx-axis and y-axis in selected respective step sizes, referred to hereinas Dx and Dy, which are typically substantially smaller than respectivePx and Py. Thus, stage 246 is configured to move detector 240 in stepsequal to a fraction of the pixel size described above.

Equations 1 and 2 below provide explicit expressions for estimating thesize of Dx and Dy, respectively:

Dx=p _(x) /m  (1)

Dy=p _(y) /n  (2)

where n and m are typically integer numbers indicative of the selectedstep size in x-axis and y-axis, respectively.

In some embodiments, processor 22 is configured to receive theelectrical signals produced by a given sensor 243, and to set therotation speed of wafer 190 in response to the received signals. Notethat the acquisition time of sensor 243 inversely depends on theintensity of the sensed X-rays. For example, if the electrical signalsreceived at a given area of wafer 190 are indicative of a relatively lowintensity of the sensed X-rays, processor 22 may instruct the controllerto decelerate the motion of detector 240 at the given area so as toincrease the flux of photons and thereby to increase the SBR sensed atthe given area.

Similarly, in case of a relatively high intensity of the sensed X-raysat different rotation angles of wafer 190, processor 22 may instruct thecontroller to accelerate the motion of detector 240 at the differentarea so as to increase the measurement throughput.

In some embodiments, processor 22 or a controller of detector 240 isconfigured to control the acquisition time so that detector 240 receiveda predefined intensity range across the measured positions on wafer 190.The predefined intensity range enables sufficient intensity to obtainhigh SBR, and yet, prevents saturation and non-linear sensing in therespective sensors of detector 240.

In some embodiments, processor 22 is configured to acquire, from a givensensor 243, an image based on the intensity of the scattered photons ofbeam 222, at an acquisition time, t. Therefore, in an array of an n-by-msub-pixels, processor 22 allocates for each sub-pixel a uniform timeinterval of t/(m×n), so as to acquire n-by-m sub-images within theacquisition time t.

In some embodiments, processor 22 is configured to move detector 240 ina raster pattern along the x and y axes using respective step sizes Dxand Dy, so as to measure the intensity distribution of each timeinterval at a different position of detector 240 spanning the total areaof a single pixel.

In some embodiments, processor 22 is configured to combine the n-by-msub-images received from the respective sensor 243, into a single pixel.Processor 22 may apply to the received sub-images any suitable method,such as but not limited to, a simple arithmetic interpolation, or anysuitable image processing algorithms, so as to increase the resolution(e.g., angular resolution) of the combined image.

In some embodiments, by applying the sub-pixel stepping and combiningthe n ° m sub-images to form a single image having improved angularresolution, processor 22 overcomes a resolution limitation of SAXSsystems caused by the available pixel size of the respective detectorassembly.

Equation (3) below provides an expression for calculating the angularresolution A of a detector having a pixel size p, positioned at adistance d from the wafer in question:

Δθ=p/d  (3)

Based on a typical pixel size of 172 um a distance of about 5-6 metersis required to obtain an angular resolution on an order of 0.3 mrad-0.5mrad.

In some embodiments, by using the sub-pixel stepping and combining then×m sub-images, as described above, the designed distance betweendetector 240 and wafer 190 may be reduced, for example by a factor ofthree, e.g., to less than two meters, while maintaining the requiredangular resolution.

In some embodiments, processor 22 is configured to reduce the overallcycle time of measuring the features in question of wafer 190 byincreasing the speed of detector 240 to the maximal level that enablesacquiring the sub-images at sufficiently-high SBR as will be describedin detail hereinbelow.

The intensity of scattered beam 222 typically depends on the Fouriertransform of the electron density distribution ρ(r) of the scatteringobjects. For weak scattering, the scattered amplitude “A” may becalculated using equation (4):

A(Q)∝∫_(V)ρ_(e)(r)exp(−iQ·r)dr  (4)

where Q is the scattering vector and is determined by X-ray wavelength λand the respective angles of incident beam 130 and scattered beam 222relative to wafer 190.

Equation (5) below provides a well-known expression for calculating thescattered intensity in the kinematical approximation:

I(Q)=(|A(Q)|)² +Ib(Q)  (5)

where Ib(Q) is an incoherent “background” intensity contribution systemof any origin such as fluorescence or scattering from structures in thewafer beyond the coherence length of the radiation or parts of the tool,i.e. slits or beam blocker.

The electron density ρ_(e) is related to a refractive index “n” of thescattering objects of wafer 190. Equation (6) below provides anexpression for calculating the refractive index n:

n=1−δ−iβ  (6)

Where δ and β are, respectively, the dispersive and absorptivecomponents of the wave-matter interaction.

Note that the value of the refractive index is close to unity for allmaterials in the range of hard X-rays, where the value of 6 is on theorder of 10⁻⁶.

Therefore, equation (7) below may be used for calculating the electrondensity ρ_(e):

$\begin{matrix}{\rho_{e} = {\frac{2\; \pi}{\lambda^{2}r_{e}}\delta}} & (7)\end{matrix}$

where r_(e) is the value of a classical electron radius, equals to2.818×10⁻¹⁵ meters.

In some embodiments, processor 22 is configured to calculate a physicalmodel comprising the topography and the materials of the features inquestion described above. Processor 22 is configured to compare betweenthe calculated and measured intensities using any suitable parameters,such as a numerical goodness-of-fit (GOF), and to adjust the modelparameters so as to minimize the different between the calculated andmeasured data.

The dataset fitted by processor 22 may comprise one or more 1D datasets,such as the intensity distribution integrated along or across thediffraction peaks for different orientations of beam 130 and/or detector240 relative to wafer 190, or a series of 2D images of the scatteredintensity patterns or a combination thereof.

As described above, processor 22 is configured to reduce the measurementtime of the features in question by acquiring data using differentacquisition times at different locations across wafer 190. In someembodiments, processor 22 may apply the different acquisition time bydetector 240 in various conditions. For example, when measuringdifferent type of features (e.g., geometrical structure and/ormaterials), and/or layout (e.g., a single feature, or a dense array offeatures), and/or the angle between beam 130 and surface 192 of wafer190, and/or the angle between beam 222 and the active surface ofdetector 240.

In some embodiments, processor 22 is configured to adjust the signalacquisition time so as to obtain sufficient intensity that enablessufficiently high SBR of the electrical signals received from detector240. The measurement uncertainty of scattered X-ray having an averageintensity based on N counts is typically dictated by Poisson countingstatistics such that the standard error is given by √N and thefractional error is given by 1/(√N). Therefore, processor 22 may reducethe measurement uncertainty by increasing the number of counts.

As described above, processor 22 may reduce the acquisition time at somelocations, where the intensity of beam 222 sensed by detector 240 ishigh, and increase the acquisition time at other locations, where theintensity of the sensed X-rays is lower, so as to obtain sufficient, butnot excess, of X-ray photon counting statistics.

In alternative embodiments, processor 22 may apply pre-processing, suchas down-sampling and principal component analysis (PCA), to rawelectrical signals received from detector 240, such as 1D intensityprofiles and/or 2D images for one or more rotation angles. Subsequently,processor 22 may apply one or more machine learning algorithms to thepre-processed data and to complementary data that can be used forassessing the value of the data, such as electrical test data of (e.g.,of the features in question).

In these embodiments, processor 22 may use any suitable types of machinelearning algorithms, such as the TensorFlow open-source machine learningframework initially developed by Google (Mountain View, Calif.), astraining decks for deep learning using neural networks.

Processor 22 may subsequently apply the trained model obtained based ona preceding dataset, to data measured on subsequent wafers 190, so as topredict the electrical performance of the respective device under test,or to provide a user of systems 10, 30 and 40, other useful attributesbased on the data measured on the subsequent wafers 190. Note that usingthe embodiments of such machine learning algorithms may require highsampling so as to develop reliable regression-based models.

In some embodiments, detector 240 comprises electronic circuitries (notshown) configured to discriminate between low-energy and high-energyphotons of beam 220. In some embodiments, processor 22 is configured toreduce the background intensity caused, for example, by X-rayfluorescence and high-energy cosmic rays.

In other embodiments, processor 22 is configured to remove many of thehigh-energy cosmic rays using software-based filters in combination withthe sub-pixel resolution enhancement described above. In theseembodiments, detector 240 may not include the hardware-based cosmic raydiscrimination described above.

Although the embodiments described herein mainly address X-ray analysisof single-crystal, polycrystalline or amorphous samples, such assemiconductor wafers, the methods and systems described herein can alsobe used in other technological of applications of arrays ofnanostructures.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art. Documents incorporated by reference inthe present patent application are to be considered an integral part ofthe application except that to the extent any terms are defined in theseincorporated documents in a manner that conflicts with the definitionsmade explicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

1. X-ray apparatus, comprising: a mount, which is configured to hold aplanar sample having a first side, which is smooth, and a second side,which is opposite the first side and on which a pattern has been formed;an X-ray source, which is configured to direct a first beam of X-raystoward the first side of the sample; a detector, which is positioned onthe second side of the sample so as to receive at least a part of theX-rays that have been transmitted through the sample and scattered fromthe pattern; an optical gauge, which is configured to direct a secondbeam of optical radiation toward the first side of the sample, to sensethe optical radiation that is reflected from the first side of thesample, and to output a signal, in response to the sensed opticalradiation, that is indicative of a position of the sample; and a motor,which is configured to adjust an alignment between the detector and thesample in response to the signal.
 2. The apparatus according to claim 1,wherein the signal is indicative of at least one position parameter,selected from a group of position parameters consisting of a distancebetween the sample and the detector and an orientation of the samplerelative to the detector.
 3. The apparatus according to claim 2, whereinthe orientation of the sample comprises an inclination angle of thesample relative to a surface of the detector.
 4. The apparatus accordingto claim 1, wherein the sample comprises a single-crystal material, andcomprising an additional detector, which is configured to measure anintensity of at least a portion of the X-rays that has been diffractedfrom a lattice plane of the single-crystal material, and comprising acontroller, which is configured to calibrate an orientation of the firstbeam of X-rays with respect to the lattice plane responsively to themeasured intensity.
 5. The apparatus according to claim 1, andcomprising a processor, which is configured to instruct the opticalgauge to direct the second beam toward multiple locations on the firstside of the sample so as to output multiple respective signalsindicative of multiple respective optical radiations reflected from themultiple locations, wherein the processor is further configured todisplay, based on the multiple signals, a three-dimensional (3D) mapindicative of the position of the sample at least at the multiplelocations.
 6. The apparatus according to claim 5, wherein the processoris configured to estimate, based on the multiple locations, one or moreadditional positions of the sample at additional one or more respectivelocations on the first side, and to display the additional locations onthe 3D map.
 7. The apparatus according to claim 1, and comprising anenergy dispersive X-ray (EDX) detector assembly, which is configured tomeasure X-ray fluorescence emitted from the pattern at the position ofthe sample, and to output an electrical signal indicative of anintensity of the X-ray fluorescence measured at the position.
 8. Theapparatus according to claim 7, wherein the EDX detector assemblycomprises a silicon-based or a germanium-based solid-state EDX detector.9. A method, comprising: holding, on a mount, a planar sample having afirst side, which is smooth, and a second side, which is opposite thefirst side and on which a pattern has been formed; directing a firstbeam of X-rays toward the first side of the sample; receiving, from adetector positioned on the second side of the sample, at least a part ofthe X-rays that have been transmitted through the sample and scatteredfrom the pattern; directing a second beam of optical radiation towardthe first side of the sample for sensing the optical radiation that isreflected from the first side of the sample, and outputting a signal, inresponse to the sensed optical radiation, that is indicative of aposition of the sample; and adjusting an alignment between the detectorand the sample in response to the signal.
 10. The method according toclaim 9, wherein the signal is indicative of at least one positionparameter, selected from a group of position parameters consisting of adistance between the sample and the detector and an orientation of thesample relative to the detector.
 11. The method according to claim 10,wherein the orientation of the sample comprises an inclination angle ofthe sample relative to a surface of the detector.
 12. The methodaccording to claim 9, wherein the sample comprises a single-crystalmaterial, and comprising measuring an intensity of at least a portion ofthe X-rays that have been diffracted from a lattice plane of thesingle-crystal material, and calibrating an orientation of the firstbeam of X-rays with respect to the lattice plane responsively to themeasured intensity.
 13. The method according to claim 9, and comprisinginstructing the optical gauge to direct the second beam toward multiplelocations on the first side of the sample so as to output multiplerespective signals indicative of multiple respective optical radiationsreflected from the multiple locations, and displaying, based on themultiple signals, a three-dimensional (3D) map indicative of theposition of the sample at least at the multiple locations.
 14. Themethod according to claim 13, and comprising estimating, based on themultiple locations, one or more additional positions of the sample atadditional one or more respective locations on the first side, anddisplaying the additional locations on the 3D map.
 15. The methodaccording to claim 9, and comprising measuring X-ray fluorescenceemitted from the pattern at the position of the sample, and outputtingan electrical signal indicative of an intensity of the X-rayfluorescence measured at the position of the sample.
 16. X-rayapparatus, comprising: a mount, which is configured to hold a samplecomprising a single-crystal material and having a first side and asecond side, which is opposite the first side; an X-ray source, which isconfigured to direct a beam of X-rays toward the first side of thesample; a detector, which is positioned on the second side of the sampleand is configured to receive at least a portion of the X-rays that havebeen diffracted from a lattice plane of the single-crystal material; amotor, which is configured to adjust an alignment between the detectorand the sample; and a controller, which is configured to measure anorientation of the sample relative to the detector based on thediffracted X-rays and to drive the motor to adjust the alignmentresponsively to the measured orientation.